The major objective will be to elucidate the molecular structure of the allotypic sites of human histocompatibility antigens. To this end, highly purified antigens expressing different phenotypes will be selectively cleaved with proteases and antigenically reactive peptides will be isolated by affinity chromatography with monoclonal antibodies. Such peptides will be analyzed for molecular size and amino acid sequence to determine their location within the overall molecular structure of histocompatibility antigens and to delineate the specific molecular structures which are associated with known allotypic specificities of human histocompatibility antigens.